<jats:p>Podostemaceae of the Phou Khao Khouay National Park and adjoining areas (PKK Area) in Lao PDR were recorded in separate papers, and scattered data make it difficult to gain the whole picture of their diversity and evolution. Our literature research enumerated 18 species and six genera, which account for 43% of species and 55% of genera indigenous to Laos, including 13 PKK Area-endemic species (72%) and three PKK Area-endemic genera (50%). The richness and endemism of species and genera of the PKK Area have the highest density in Asia. Most likely, the genera and all nine endemic species of Hydrobryum evolved in the PKK Area. Four endemic species of Hydrobryum are ribbon-like/subcylindrical-rooted and sympatric with foliose-rooted congeners. The evolutionary way is opposite to the early one in the ancestor of the Hydrobryum clade; the 2-way evolution, together with sympatry, is suggested to be non-adaptive. Two species are perhaps polyphyletic in the initial stage of phylogenetic-species transition. The characteristics of the Podostemaceae found in the PKK Area are important in the context of both biological science and conservation.</jats:p>